Transcript Document
Lifting and Cooling Four major processes make clouds form Adiabatic Cooling Mixing Lifting Advective Cooling Warm air can hold more moisture than cold air Adiabatic COOLING a change in temperature due solely to a change in altitude dry adiabatic lapse rate= 10c/km Moist adiabatic lapse rate= 6C/km The reason for the difference is that latent heat is released when water condenses water's high heat of vaporization creates a significant release of the energy when it condenses (and is an important source of energy in the development of thunderstorms) Atmospheric Lifting For clouds to form air must lift 3 lifting mechanisms Convectional Orographic Frontal Convectional Lifting Some areas of Earth's surface pick up heat better than others. As air warms it becomes less dense and rises. As it rises it cools and sinks Circulatory motion is called convectional lifting If cooling occurs close to the air's saturation temperature, condensing moisture forms a cumulus cloud Form and dissipate over the same area Orographic Lifting An air mass lifts when it is pushed upward over an obstacle such as a mountain range As rising air cools, if it is humid, it forms a cloud. If stable air stratus If unstable air cumulus As air moves down the other slope it warms The descending air is dry because the moisture was removed on the other side This is called the Rain Shadow Effect. Example: Eastern Colorado and western Kansas See figure 27.10 Frontal Lifting When fronts collide warm less dense air is forced up over cold dense air Fronts—dryline The source region for cT air is the desert Southwest, the high plains and Mexico with relation to the United States. The air has low dewpoints and warm to hot afternoon temperatures but with mild nighttime temperature. Skies are generally clear in cT air. This allows daytime heating during the day and radiational cooling at night. The cT air mass is most prevalent in summer; in the cool season it is not as discernible. Due to the buoyancy and elevation of cT air across North America, this air will advect into the midlevels of the atmosphere once it moves out of its source region. This creates a cap of mild dry air. If this air advects over PBL mT air, the severe thunderstorm threat increases significantly. The boundary of cT is most noticeable with the creation of a dryline. A dryline separates mT air from cT air. Depending on the strength of the dryline, convergence along the dryline and the dynamics above the dryline, severe thunderstorms can form near a dryline boundary. Advective Cooling The process by which the temperature of an air mass decreases as the air mass moves over a cold surface Cold ocean Land surface Precipitation Any moisture that falls from the air to Earth’s surface Four major types Rain Snow Sleet Hail Rain Classified by droplet diameter Fine mist to large droplets Measured with a rain gauge Here is a tipping bucket rain gauge Sleet Ice pellets Or ice glaze Does not freeze till it hits a cold surface This is commonly called an ice storm